Two phenomena known to inhibit the potential habitability of planets -- tidal forces and vigorous stellar activity -- might instead help chances for life on certain planets orbiting low-mass stars, University of Washington astronomers have found.
In a paper published this month in the journal Astrobiology, UW doctoral student Rodrigo Luger and co-author Rory Barnes, research assistant professor, say the two forces could combine to transform uninhabitable "mini-Neptunes" -- big planets in outer orbits with solid cores and thick hydrogen atmospheres -- into closer-in, gas-free, potentially habitable worlds.
Most of the stars in our galaxy are low-mass stars, also called M dwarfs. Smaller and dimmer than the sun, with close-in habitable zones, they make good targets for finding and studying potentially habitable planets. Astronomers expect to find many Earthlike and "super-Earth" planets in the habitable zones of these stars in coming years, so it's important to know if they might indeed support life.
Super-Earths are planets greater in mass than our own yet smaller than gas giants such as Neptune and Uranus. The habitable zone is that swath of space around a star that might allow liquid water on an orbiting rocky planet's surface, perhaps giving life a chance.
"There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets," Luger said. "Two important ones are strong tidal effects and vigorous stellar activity."
A tidal force is a star's gravitational tug on an orbiting planet, and is stronger on the near side of the planet, facing the host star, than on the far side, since gravity weakens with distance. This pulling can stretch a world into an ellipsoidal or egglike shape as well as possibly causing it to migrate closer to its star.
"This is the reason we have ocean tides on Earth, as tidal forces from both the moon and the sun can tug on the oceans, creating a bulge that we experience as a high tide," Luger said. "Luckily, on Earth it's really only the water in the oceans that gets distorted, and only by a few feet. But close-in planets, like those in the habitable zones of M dwarfs, experience much stronger tidal forces."
This stretching causes friction in a planet's interior that gives off huge amounts of energy. This can drive surface volcanism and in some cases even heat the planet into a runaway greenhouse state, boiling away its oceans, and all chance of habitability.
Vigorous stellar activity also can destroy any chance for life on planets orbiting low-mass stars. M dwarfs are very bright when young and emit lots of high-energy X-rays and ultraviolet radiation that can heat a planet's upper atmosphere, spawning strong winds that can erode the atmosphere away entirely. In a recent paper, Luger and Barnes showed that a planet's entire surface water can be lost due to such stellar activity during the first few hundred million years following its formation.
"But things aren't necessarily as grim as they may sound," Luger said. Using computer models, the co-authors found that tidal forces and atmospheric escape can sometimes shape planets that start out as mini-Neptunes into gas-free, potentially habitable worlds.
How does this transformation happen?
Mini-Neptunes typically form far from their host star, with ice molecules joining with hydrogen and helium gases in great quantity to form icy/rocky cores surrounded by massive gaseous atmospheres.
"They are initially freezing cold, inhospitable worlds," Luger said. "But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration." This process can bring mini-Neptunes into their host star's habitable zone, where they are exposed to much higher levels of X-ray and ultraviolet radiation.
This can in turn lead to rapid loss of the atmospheric gases to space, sometimes leaving behind a hydrogen-free, rocky world smack dab in the habitable zone. The co-authors call such planets "habitable evaporated cores."
"Such a planet is likely to have abundant surface water, since its core is rich in water ice," Luger said. "Once in the habitable zone, this ice can melt and form oceans," perhaps leading to life.
Barnes and Luger note that many other conditions would have to be met for such planets to be habitable. One is the development of an atmosphere right for creating and recycling nutrients globally.
Another is simple timing. If hydrogen and helium loss is too slow while a planet is forming, a gaseous envelope would prevail and a rocky, terrestrial world may not form. If the world loses hydrogen too quickly, a runaway greenhouse state could result, with all water lost to space.
"The bottom line is that this process -- the transformation of a mini-Neptune into an Earthlike world -- could be a pathway to the formation of habitable worlds around M dwarf stars," Luger said.
Will they truly be habitable? That remains for future research to learn, Luger said.
"Either way, these evaporated cores are probably lurking out there in the habitable zones of these stars, and many may be discovered in the coming years."
Luger is lead author of the paper, with Barnes and Victoria Meadows his UW co-authors. Other co-authors are E. Lopez and Jonathan Fortney of the University of California, Santa Cruz, and Brian Jackson of Boise State University.
The research was done through the Virtual Planetary Laboratory, a UW-based interdisciplinary research group, and funded through the NASA Astrobiology Institute under Cooperative Agreement Number NNA13AA93A .
This release is based on an essay by Luger. View a poster for the research. For more information, contact Luger at 206-543-6276 or email@example.com; or Barnes at 206-543-8979 or firstname.lastname@example.org.
Peter Kelley | EurekAlert!
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
28.04.2017 | Event News
20.04.2017 | Event News
18.04.2017 | Event News
28.04.2017 | Medical Engineering
28.04.2017 | Earth Sciences
28.04.2017 | Life Sciences